Surface thermocouple

ABSTRACT

A surface thermocouple assembly including a metal sheath having thermocouple conductors extending therethrough in spaced relation from each other and from the sheath by electrical insulating material, an opening in the sheath wall at the hot junction end and through which the conductors are extended, a ceramic plug receiving the conductors and mounted in the opening, a fusion weld of the conductors outside the plug, and a V-shaped pad with a knife-shaped edge integrally formed at the junction end. The thermocouple is made from a length of sheathed thermocouple conductors by forming a keyhole slot at the junction end, removing the insulation around the conductors, bending the conductors through the opening of the keyhole slot, mounting a ceramic plug in place, fusion welding the ends of the conductors outside the plug to define the thermojunction, inserting a metal end plug at the junction end, welding closed the slot and the end plug in position, and building up a V-shaped welded pad at the hot junction end. Thereafter, mounting of the thermocouple assembly onto a surface for which the temperature is to be monitored includes the positioning of the hot junction end onto the tube with the knife-shaped edge of the pad in engagement with the tube, and providing void-free welds between the pad and the tube such that the distance between the thermojunction and the tube surface is substantially less than the distance between the junction and the closest free surface of the mounting welds.

This is a division, of application Ser. No. 447,961, filed Mar. 4, 1974,now U.S. Pat. No. 3,874,239.

This invention relates in general to a thermocouple assembly, and moreparticularly to a thermocouple assembly for measuring surfacetemperatures, and still more particularly to a surface thermocoupleassembly for measuring the temperatures of heat exchanger tubes forheaters or furnaces, and still more particularly to a method of makingan improved surface thermocouple, and still more particularly to animproved surface thermocouple assembly in combination with a tube of aheat exchanger and the method of mounting the assembly on the tube.

The thermocouple assembly of the present invention is primarily intendedfor obtaining accurate temperature measurement of fired heat exchangertubes in order to provide optimum safe and efficient operation. Forexample, it is important to accurately measure the tube skin or surfacetemperature of heat exchanger tubes in heaters for refineries wherepetroleum is being heated for refining operations in order to maintainoptimum product flow through the heat exchanger at all times, andprevent tube rupture or damage. Specifically, obtaining accurate tubewall temperature measurements permits the operator to adjust theoperating conditions of the heater so that maximum allowabletemperatures are not exceeded while at the same time obtaining maximumproduct flow through the heat exchanger even with the existence of tubefouling.

Many factors make it difficult to obtain accurate temperatures ofrefinery heater tubes with the use of thermocouples since thethermocouples must be mounted on the tubes where high temperature gasesand flame are encountered. These gases and flame may be up to 600° to800° F. hotter than the heater tube at the point of measurement. Itshould be appreciated the gases and flame are deleterious to thematerials of the thermocouple's assemblies and heat transferred alongthe assemblies to the point of attachment by the gases and flame tend tocause the thermocouple to be responsive to the temperature of thesegases and flame rather than the tube wall. These situations prevent theefficient product flow rate through the tubes.

Heretofore, thermocouple assemblies used for monitoring tube walltemperatures of heat exchangers have not been sufficiently accurate andreliable due to the design and the manner in which the assemblies havebeen mounted on the tube walls. Also, insufficient protection has beengiven to known thermocouple assemblies to withstand the gases and flamesin a heater.

Surface thermocouples such as those illustrated in U.S. Pat. No.1,140,701 and 2,607,808 have not been satisfactory since they do notproperly isolate the thermocouple conductors from the combustion gasesof the heaters. Accordingly, early failure and inaccurate temperaturemonitoring is experienced.

It has been proposed that shielding of thermocouple assemblies protectsthermocouple conductors and the thermojunction from the effects of gasesand flame, as in U.S. Pat. No. 2,048,680 and 2,048,681, but sucharrangements have not been satisfactory because of difficulties inobtaining leakproof welds at the point of attachment of the shield tothe tube. Further, the shield being in the form of a tubular member isnecessarily massive and of a large physical size to accommodate theceramic insulators and thermocouple conductors such that it is difficultto provide expansion loops, and further a "shadow" is cast on the tubeby the large shield, preventing the absorption of part of the radiantand conductive energy. Moreover, the interior of the tubular shieldhaving a temperature gradient therealong causes convection of gaseswhich draws in contaminates such as refinery gases and moisture thattends to deteriorate the conductors and thermojunction.

One specific pad-type thermocouple assembly heretofore known andillustrated in U.S. Pat. No. 3,143,439 is unsatisfactory since the padis secured to the tube wall by depositing a weld bead at the peripheryof the pad which causes arching and separation of the pad from the tubewall surface. This creates a void or gas gap that is thermallyresistive, thereby causing the hot junction of the assembly tube moreresponsive to the gases and flame temperature and less responsive to thetube wall temperature, which results in a temperature readout higherthan that of the tube wall. Ultimately, this would in turn preventoperating the heater with the optimum product flow through the heatexchanger, thereby decreasing the overall efficiency of the heatexchanger.

The present invention avoids the problems heretofore known in providinga surface thermocouple assembly which obtains the maximum thermalconductivity between the hot junction or sensing end of the assembly andthe tube wall of a heat exchanger and at the same time minimizingthermoconductivity between the hot junction and the gases and flame.Accordingly, the thermocouple will respond to the actual tube walltemperature and not the gases temperature to provide an accurate andreliable temperature readout so that the heater can be operated toprovide maximum product flow and efficiency while at the same timepreventing tube damage or failure. The present invention creates a longheat path between the gases and thermocouple hot junction and a shorterdirect heat path between the hot junction and the tube wall.

The thermocouple assembly of the invention includes sheathedthermocouple conductors between the cold junction or reference end andthe hot junction or sensing end and a pad formed on the hot junction endintegrally with the sheath and so the pad can be thereafter attached bywelding to a heater tube wall with void-free welding such that thedistance between the thermojunction and the tube wall is substantiallyless than the distance between the thermojunction and the nearestsurface subjected to the high temperature gases and flame. The assemblyis made by performing operations on a length of insulated and sheathedthermocouple conductors in a manner hereinafter described. The shape ofthe pad is unique and such as to permit the proper mounting to a tubewall to accomplish the objectives of obtaining long life, accurate andreliable operation.

It is therefore an object of the present invention to provide a new andimproved surface thermocouple assembly for monitoring the temperature ofheat exchanger tubes and particularly tubes heated by combustion gases.

Another object of this invention is in the provision of an improvedsurface thermocouple assembly for heat exchanger tubes which provideslong life, accurate and reliable operation, together with ease ofinstallation so that such operation can be obtained.

A still further object of the invention resides in the provision of animproved thermocouple assembly which may be installed on heat exchangertubes with relative ease by conventional welding operations.

Still another object of the invention is to provide an improvedthermocouple assembly and a method of making the assembly wherein thethermocouple materials of the assembly are protected against exposure todeleterious combustion gases and where the assembly is responsive totube wall temperature and not influenced by surrounding gastemperatures.

A still further object of the invention is to provide a thermocoupleassembly for monitoring heat exchanger tubes wherein the thermocoupleconductors are protected by a sheath and compacted insulating materialto resist combustion gas corrosion and which will retain integrity whensuspended from its point of attachment to a tube to its point of exitthrough the heater wall under firing conditions.

Another object of the invention is to provide a thermocouple assemblyincluding a lead section which can be formed or bent during installationwithout effecting its operation thereafter and which may be formed forpermitting relative movement between the point of attachment to a heatexchanger tube and the exit point from the heat exchanger enclosure.

A still further object of the invention is in the provision of animproved surface thermocouple assembly and the mounting of the assemblyon a heater tube where the heat path between the tube wall surface andthe thermojunction or hot junction is short and direct and the heat pathbetween the thermojunction and the surrounding gases is lengthy andindirect and particularly where the heat path between the thermojunctionand the outer wall is substantially less than that between thethermojunction and the combustion gases.

Other objects, features and advantages of the invention will be apparentfrom the following detailed disclosure, taken in conjunction with theaccompanying sheets of drawings, wherein like reference numerals referto like parts, in which:

FIG. 1 is a fragmentary sectional view taken through a heater or furnaceand a heat exchanger tube having the thermocouple assembly of thepresent invention mounted on the tube and diagrammatically connected toa temperature readout instrument;

FIG. 2 is a fragmentary side elevational view of the heat exchanger tubethermocouple assembly taken generally along line 2--2 of FIG. 1;

FIG. 3 is a top plan view of the thermocouple assembly of the inventionmounted on the heat exchanger tube and taken generally along line 3--3of FIG. 1;

FIG. 4 is an enlarged perspective view of the thermocouple assemblymounted on a heat exchanger tube according to the embodiment of FIG. 1;

FIG. 5 is an enlarged side elevational view of the form of thermocoupleassembly of FIG. 1 illustrating diagrammatically the relative positionof the thermojunction or hot junction in the hot junction or sensing endof the assembly;

FIG. 6 is a longitudinal sectional view of the sensing end of theassembly taken substantially from the encircled area of FIG. 5;

FIG. 7 is an enlarged end elevational view of the hot junction end ofthe assembly and a fragmentary section of the heat exchanger tubeillustrating the void-free weldments securing the hot junction end ofthe assembly to the tube wall;

FIG. 8 is a diagrammatic illustration of the welding details for weldingthe hot junction end to the heat exchanger tube wall;

FIG. 9 is an enlarged end elevational view of a completely formed hotjunction end of the thermocouple assembly of the present inventionillustrating the structure of the welding surfaces;

FIG. 10 is an enlarged diagrammatic view of a heat exchanger tubeillustrating the various positions for the hot junction end of awraparound assembly according to the invention;

FIG. 11 illustrates diagrammatically another form of mounting athermocouple assembly on a heat exchanger tube with an expansion loopand a near side junction location;

FIG. 12 is a view similar to FIG. 1 but illustrating a typical straightjunction mounting of an assembly;

FIG. 13 is a fragmentary enlarged perspective view of the axialinstallation shown in FIG. 12;

FIG. 14 is a fragmentary sectional view taken of the straight junctiontype of assembly shown in FIGS. 12 and 13;

FIGS. 15 to 31 illustrate a method of making the thermocouple assemblyaccording to the invention and a modification thereof as follows:

FIG. 15 is a partly sectional broken side elevational view of a lengthof sheathed and insulated thermocouple conductors, illustrating aportion of the sheath and insulation stripped from one end thereof;

FIG. 16 is an enlarged sectional view taken axially through one endillustrating the formation of a keyhole slot and the removal ofinsulation around the conductors;

FIG. 17 is a bottom plan view of the sheath with the keyhole slot;

FIG. 18 is an end elevational view of the sheath with the keyhole slot;

FIG. 19 is an axial sectional view illustrating the conductors bent toextend through the hole in the keyhole slot with the ceramic pluginstalled, additional insulation material compacted around the bends ofthe conductors, a fusion weld on the conductors to form thethermojunction, an end plug press fit into the end of the sheath andwelded into position with the keyhole slot filled with weld;

FIG. 20 is an end elevational view of the formation shown in FIG. 19;

FIG. 21 is a detailed elevational view of the hot junction endillustrating the bending of the hot junction end for defining awraparound assembly although this step would be omitted if the assemblyis to be of the straight junction type;

FIG. 22 is an elevational view of the filler rod employed to build upthe pad;

FIG. 23 is a side elevational view of the hot junction end showing thefiller rod in position on the sheath and initially welded thereto;

FIG. 24 is a diagrammatic illustration of the welds for initiallyattaching the filler rod to the sheath;

FIG. 25 is a view like FIG. 23 showing the further weld buildup betweenthe filler rod and the sheath;

FIG. 26 is a view like FIG. 24 illustrating diagrammatically the buildupwelds along the sheath and filler rod;

FIG. 27 is an end elevational view of the hot junction end illustratingthe V-shape of the pad and the knife edge defined;

FIG. 28 is a partly sectional fragmentary view of the reference or coldjunction end of the assembly to illustrate the sealing of the sheathrelative the thermocouple conductors;

FIG. 29 is an elevational view like FIG. 22 showing a modified fillerrod with conductor holes and a milled slot for bringing thethermojuction closer to and substantially right at the knife edge of thepad;

FIG. 30 is a longitudinal sectional view like FIG. 19 showing the fillerrod modification of FIG. 29; and

FIG. 31 is a diagrammatic showing of the comparative locations of thethermojunction relative to the knife edge of the pad for the embodimentsof FIGS. 27 and 30.

Referring now to the drawings and particularly to FIGS. 1 to 3, atypical installation of a thermocouple assembly according to theinvention is shown for a furnace or heater. It should be appreciatedthat the assembly of the invention is primarily intended for monitoringtemperatures of heat exchanger tubes so that the heat exchanger can beoperated at optimum efficiency and so that damage or injury to the heatexchanger tubes because of overheating can be avoided. Therefore, thethermocouple assembly as associated with a heater tube provides maximumsafety and efficiency, and especially since accurate and reliabletemperature monitoring operations are performed by the assembly of theinvention. It should be further appreciated that the heat exchangertubes are exposed to high temperature combustion gases and flame whilecarrying a suitable product to be heated.

While the assembly of the invention is shown generally in connectionwith the temperature monitoring of a single heat exchanger tube, itshould be appreciated that any number of thermocouple assemblies may beutilized in a single heater or furnace for monitoring temperatures oftubes at desired locations.

In FIGS. 1 to 3, the furnace or heater includes a wall 35 defining acombustion chamber 36 in which a heat exchanger 37 is located asrepresented by a single tube 38. The thermocouple assembly according tothe invention in the form illustrated in FIGS. 1 to 3 is generallyindicated by the numeral 40 and includes sheathed thermocoupleconductors 41 with a hot junction or sensing end 42 and a cold junctionor reference end 43. As illustrated, the hot junction end is mounted onthe outer surface of the tube 38, while the sheath of the thermocoupleextends through the furnace wall 35, and the cold junction end 43 isconnected to a terminal block and head assembly 44 that is in turnconnected to a temperature readout instrument 45. The exit point of thethermocouple assembly at the furnace wall may have a sliding fit tohandle longitudinal movement caused by expansion and contraction of theheat exchanger or may be sealed and fixed where an expansion loop 46 isprovided in the thermocouple assembly to handle relative movement of theheat exchanger tube. Hence, it will be understood that if the expansionloop is not provided, movement of the heat exchanger tube may be handledat the furnace wall. While the hot junction end 42 of the thermocoupleassembly is welded in place on the heater tube, it may be additionallyfastened to the heater tube by means of a retaining clip 47.

The installation illustrated in FIGS. 1 to 3 is in the form of awraparound thermocouple assembly where a portion of the thermocouplewraps around the heater tube. In this illustration the assembly iswrapped around ninety degrees. However, it should be appreciated thatthe assembly may be wrapped around to a greater or lesser extent asshown by the positions in FIG. 10. It should be further appreciated thatthe hot junction end of the thermocouple assembly may be positioned atthe far side of the tube, as shown in FIGS. 1 to 3, or at the near side,as illustrated in FIG. 11. In general, the hot junction end will bepositioned at a hot spot of the tube.

It should be further appreciated that the thermocouple assembly of theinvention, instead of being formed for wraparound mounting as in FIGS. 1to 4, may be used in a typical straight junction installation asillustrated in FIGS. 12 and 13. Here the assembly is identified by thenumeral 40A where the hot junction end 42A is arranged axially withrespect to the heater tube 38A. The cold junction end 43A is alsoconnected to a suitable terminal block and head assembly outside thefurnace wall 35.

Referring now particularly to FIGS. 5, 6, 7, 8 and 9, the hot junctionend 42 of the thermocouple assembly includes a thermojunction or hotjunction 50 defined by the joining of the ends of the thermocoupleconductors 51. Any suitable combination of metals may be provided forthe conductors, such as iron-constantan, chromel-alumel, or the like.The conductors are arranged within a tubular metal sheath 52 andelectrically insulated from each other and from the sheath by means of asuitable ceramic insulation 53. The ceramic insulation may be of anysuitable type, such as magnesium oxide, aluminum oxide or beryliumoxide, which is initially provided in granular or powdered form butcompacted in the sheath so that the conductors are fully supported ininsulated relation to each other and to the sheath. Moreover, thecompacted insulation eliminates any space for passage of gases withinthe sheath which could be deleterious to the conductors and thejunction.

The hot junction end of the assembly is closed to define an end wall 54and the conductors are bent substantially perpendicular relative to theaxis of the sheath through an opening in the sheath wall and maintainedin spaced relation by a hard ceramic junction insulating plug 55. Themanner in which the hot junction end is formed will be describedhereinafter. The hot junction 50 is formed by fuse-welding the ends ofthe thermocouple conductors together outside the ceramic plug 55. A pad56 is provided at the hot junction end which also is suitably formed, aswill be described hereinafter, to provide suitable weld surfaces forfacilitating the welding of the hot junction end to the heater tube. Asparticularly seen in FIG. 9, the pad 56 is generally V-shaped in crosssection and includes inclined weld surfaces 57, 57 merging to asubstantially knife edge 58 which is first placed into engagement withthe tube at the commencement of the welding operations. The knife-shapededge 58, as illustrated in FIGS. 7 and 8, abuts directly against theskin or surface of the tube 38 to which the hot junction end is to beconnected. This brings the hot junction 50 in close proximity to thetube wall. Following the positioning of the hot junction end on a tubesurface, conventional welding steps result in securing the thermocoupleto the tube. As seen in FIG. 9, the weld surfaces 57 are angularlyspaced apart about 90° so that when the assembly is positioned inrelation to a surface onto which it is to be welded in an erect manner,the weld surfaces are angularly related to the tube surface about 45°.This is considered a substantially ideal arrangement for facilitatingwelding of two parts together.

As illustrated in FIG. 8, the welding operations include initial or rootweld passes 60, 60 between the weld surfaces 57 and the tube surface 38.Thereafter, secondary weld passes 61, 61 are produced between the weldsurface 57 of the thermocouple assembly and the tube surface 38.Thereafter, final weld passes 62, 62 and 63, 63 complete the weldment.Care must be taken during the welding operations to preclude any voidsor gas gaps in the weldment.

It will be appreciated that a solid metal heat path is produced betweenthe hot junction 50 and the skin of the tube 38 by the thermocoupleassembly and the manner in which it is attached to the tube wall.Further, the distance between the thermojunction and the skin of theheater tube, as indicated in FIG. 8 at 65, is substantially less thanthe distance between the thermojunction and the nearest receptor surfaceor surface where combustion gases impinge as represented by 66 and 67.It will be appreciated here the distances axially of the assembly to thenearest receptor surface, as indicated by the distances 68 and 69 inFIG. 5, are much greater than the distances 66 and 67 shown in FIG. 8.Accordingly, the comparative distances of concern may be bestappreciated from FIG. 8. While the distance between the junction and thetube wall is substantially less than the shortest distance between thejunction and a receptor surface, it can be appreciated that it issubstantially less than half and less than about one-quarter.

The short direct path between the thermojunction and the tube wallprovides a direct flow of heat energy therebetween, while the hottercombustion gases essentially bypass the junction by going along thesheath weld deposits to the tube wall. This minimizes the effect of thehot gases on the junction and results in making the hot junctionresponsive to the temperature of the tube wall and not the gases.Accordingly, a more reliable and accurate temperature monitoringoperation of the tube can be obtained by the present invention.

It can be further appreciated that in order to permit the welding of theassembly to a tube wall without producing any voids between the sheathand tube, there must be provided suitable weld surfaces on thethermocouple assembly pad to enable such welding operations. It can beappreciated that any void provided in the weld would interrupt the heattransfer path. The length of the pad on the assembly is such that thethermojunction will be spaced sufficiently from the nearest receptorsurface at the opposite ends of the pad.

While the details of a thermocouple assembly which is formed at the hotjunction end is illustrated in FIG. 6, such an assembly that is straightis illustrated in FIG. 14. However, it will be appreciated that thefunction of the formed and the straight assemblies do not differ. Itshould be further recognized the material used for the tubular sheathwill have high temperature strength. For example, it may be stainlesssteel and, more specifically, Inconel. Likewise, the weld material ofthe pad would be of a like material. Therefore, a short direct heattransfer path is defined between the hot junction and the tube surface,while a long indirect heat transfer path is defined between the hotjunction and the combustion gases. And because the hot junction is sonear to the tube wall, there will be no differences in temperaturebetween the tube wall and at the point of the hot junction.

The method of making the thermocouple assembly of the invention ispictorially illustrated in FIGS. 15 to 28. A length of sheath materialor bulk, as shown in FIG. 15, is first prepared by stripping the sheathand insulation from one end. The length of sheath material, generallydesignated by the numeral 75, includes a tubular metal sheath 76enclosing thermocouple conductors 77 arranged in spaced and insulatedrelation from each other and from the sheath by compacted ceramicinsulation 78. Stripping of sheath and insulation from the conductor 77at the one end defines leads 79 which will be at the cold junction orreference end of the assembly, as will be more apparent hereafter.

The other end of the sheath material is then prepared for defining thehot junction end by first forming a keyhole slot 80 with a slot portion81 and a hole portion 82. Ths slot extends axially of the sheath.Following the removal of insulation from the sheath around theconductors along the keyhole slot, the conductors are bent through theslot until they are in the hole or seat portion 82, as somewhat shown inFIG. 19. It should also be appreciated that preferably the slot isformed along the sheath at a point aligning with a plane extendingthrough the spaced conductors so that the conductors, as appearing inFIG. 18, are in superposed relation. A ceramic insulator or a plug 83ais then fitted over the conductors and in the insulator seat 82 formaintaining the conductors in spaced relation through the sheath wall,and insulated from each other and from the wall. The ends of theconductors may be treated so that a small portion extends beyond theceramic plug 83a for fusion welding to form the junction 83b.

The cavity within the sheath above the ceramic plug 83a is filled withpowdered ceramic insulating material such as magnesium oxide andcompacted. A solid end plug 83 is inserted in the end of the sheath inpress-fit relation against the added insulation so that the inner end isin slightly spaced relation from the bends of the conductors. The outerend of the plug extends slightly from the end of the sheath as seenparticularly in FIG. 19. The slot 81 is then closed with a weld and afillet weld is made between the end of the sheath and the end plug,thereby welding the end plug in place. No voids are made in the weld inthe slot. The fillet weld, designated by the numeral 84 in FIG. 20, ismade around the end of the sheath except in the area 84a.

Thereafter, the end of the sheath with the junction is suitably formedalong an arc if desired, as shown in FIG. 21, such that the junction isat the inside of the bend. It will be appreciated that the form shown inFIG. 21 will provide a ninety degree wraparound installation, such asthat illustrated in FIG. 4, but the form could be whatever desired inorder to provide the angular wraparound. It should also be appreciatedthat if a straight junction installation is desired, such as in FIG. 13,it is not necessary at this point to form the hot junction end of theassembly as in FIG. 21.

The pad is then constructed for the hot junction end of the assembly byfirst fitting and welding a filler rod 85, as seen in FIGS. 22, 23 and24. The filler rod is arcuately formed or of the same shape as the hotjunction end and extends from the tip end of the end plug 83 inwardly adistance such that the junction 83b is about centered between theopposite ends of the filler rod. Root or initial welds 86, 86 are madealong the filler rod end sheath, as seen in FIG. 24. It should beappreciated that the filler rod fits directly against the sheath andalong the area of the thermojunction 83b.

Overlapping buildup welds 87 and 88 are made along each side between thesheath and the filler rod. Again, care is taken to prevent any voids inthe weld buildup area. Additionally, as seen in FIG. 25, a weld cap 89is provided over the end of the end plug and in the area 84a, therebycompletely sealing the end of the sheath to prevent gases from enteringthere and into the sheath. The fillet weld is essentially completed thenfor the end plug.

As seen in FIG. 27, the weld areas are then machined to define thewelding surfaces 90 and the knife-shaped edge 91 to complete the padconstruction so that the pad 92 is then ready for welding onto a heatertube. This facilitates the ease of welding the hot junction end to theheater tube. It will be appreciated that the surfaces of the welds willbe suitably cleaned to enhance further welding thereof onto a tube. Anyfurther forming of the sheath, such as bending or an expansion loop orthe like, is performed at this time.

Completing the construction of the thermocouple assembly 75, referenceis made to the cold junction end shown in FIG. 28 where a portion of theinsulation material is removed from the end of the sheath around theconductors and replaced with a suitable epoxy potting material 94 toseal the cold junction end of the sheath and prevent entrance of gasestherein. The assembly is now completed for installation in a suitableheater or furnace.

Another embodiment of the invention is disclosed in FIGS. 29 and 30 andthe method of making same. This embodiment differs from that previouslydescribed in the construction and placement of the thermojunction. Whilethis embodiment is shown in connection with a typical straight junctionthermocouple assembly, it should be appreciated that it could be aformed unit as disclosed in FIG. 28. Rather than use a filler rod like85, shown in FIG. 22, a filler rod 100 is employed which includes amilled slot 101 extending axially of the rod and in alignment with apair of diametrically extending conductor holes 102.

As seen particularly in FIG. 30, during the making of an assemblyaccording to this embodiment, following the steps of bending thethermocouple conductors 77 through the keyhole slot, the seating of theceramic plug 83a, the insertion of the end plug 83 and the welding inthe keyhole slot and fillet welding of the end plug to the sheath, themodified filler rod 100 is positioned against the sheath so that thethermocouple conductor 77 extends through the conductor holes 102. Thesize of the holes 102 is such that the conductors will easily extendtherethrough. With the filler rod 100 in position root welds are madealong each side of the filler rod between it and the sheath to fastenthe filler rod in position. A fusion weld to form the thermojunction103, FIG. 30, is made in the milled slot 101. During the fusion weld anoxide forms on that part of the conductors extending through the fillerrod to essentially electrically insulate the conductors from the fillerrod in the area of the conductor holes 102. The depth of the milled slotis about one-third the diameter of the filler rod and the width andlength is such as to slightly clear the conductor holes.

It can be appreciated that the thermojunction or hot junction in thisembodiment is spaced even closer to the tube wall once the pad iscompletely constructed, as illustrated in FIG. 31. Indeed, the junctionis essentially directly on the skin of the tube so that it will providea most accurate and reliable temperature monitoring operation. Moreover,positioning of the thermojunction directly against the tube skin movesthe junction further from the combustion gases.

It will be understood that modifications and variations may be effectedwithout departing from the scope of the novel concepts of the presentinvention, but it is understood that this application is to be limitedonly by the scope of the appended claims.

The invention is hereby claimed as follows:
 1. The method of making athermocouple assembly from a bulk length of sheathed thermocoupleconductors wherein the bulk length includes a tubular metal sheathhaving a pair of thermocouple conductors therein held in spaced relationfrom each other and from the sheath by electrical insulating material,said method comprising the steps of forming a keyhole slot in the sheathat one end thereof aligned with the sheath axis with the hole of theslot spaced from the end of the sheath, removing the insulation from thesheath in the area of the slot, bending the conductors through the slotto a position at right angles to the sheath axis and so the conductorsextend through the slot hole, packing insulation in the sheath aroundthe bend of the conductors, inserting a ceramic plug with conductorholes over the ends of the conductors and fitting the plug in the holeof the slot thereby supporting the portions of the conductors extendingthrough the hole in electrically insulated relation to each other and tothe sheath, press fitting a solid metal plug into the end of the sheathat the slot and up against the insulation inserted at the bend of theconductors, fusion welding the conductors together outside the ceramicplug to form a thermoelectric junction, welding the metal plug to thesheath including filling the slot with weld, and forming a V-shapedwelded pad on the sheath over the junction without voids in the weldmaterial.
 2. The method of claim 1, wherein the step of forming the padincludes positioning a length of filler rod along the sheath alignedwith the junction and extending from the sheath end to a distance beyondthe junction about equal to the distance the junction is spaced from thesheath end, effecting a root weld at each side of the filler rod to weldthe rod to the sheath, and effecting further buildup welds along theroot weld such that a knife edge V-shaped pad can be formed.
 3. Themethod of claim 1, wherein the step of forming the pad includespositioning a length of filler rod along the sheath aligned with thejunction and extending from the sheath end to a distance beyond thejunction about equal to the distance the junction is spaced from thesheath end, effecting a root weld at each side of the filler rod to weldthe rod to the sheath, effecting further buildup welds along the rootweld and sheath, and machining the welds and rod to define two surfacesextending generally tangentially to the sheath and merging at a knifeedge.
 4. The method of claim 1, wherein the step of forming the padincludes preparing a length of filler rod for application to the sheathby drilling diametrically extending conductor holes in the rod forreceiving the conductors and milling a slot along one side of the rodabout the holes, positioning the rod against the sheath with theconductors extending through the holes and the milled slot facing awayfrom the sheath, fusion welding the conductors in the milled slot todefine a thermoelectric junction, effecting a root weld at each side ofthe filler rod to weld the rod to the sheath, and effecting furtherbuildup welds along the root weld such that a knife edge V-shaped padcan be formed.
 5. The method of claim 1, wherein the step of forming thepad includes preparing a length of filler rod for application to thesheath by drilling diametrically extending conductor holes in the rodfor receiving the conductors and milling a slot along one side of therod about the holes, positioning the rod against the sheath with theconductors extending through the holes and the milled slot facing awayfrom the sheath, fusion welding the conductors in the milled slot todefine a thermoelectric junction, effecting a root weld at each side ofthe filler rod to weld the rod to the sheath, effecting further buildupwelds along the root weld and sheath, and grinding the welds and rod todefine two surfaces extending generally tangentially to the sheath andmerging at a knife edge.
 6. The method of claim 1, wherein the step ofpress fitting the metal plug into the end of the sheath includespreparing the plug so that it will project slightly from the end of thesheath when in proper position, and the step of welding the plug to thesheath includes welding at the end of the sheath and around the end ofthe plug.
 7. The method of claim 1, further including the step ofstripping a portion of the conductors at the end of the sheath oppositethe junction end, removing a part of the insulation from the sheath andreplacing same with an insulating potting material to seal the sheath.